212 PHYTOHORMONES 



lum they were without effect. Both Miiller and Laibach 

 (1933) also used, instead of lateral buds, the cotyledonary 

 buds in legume seedlings, and these also were inhibited by 

 appUcation of orchid poUinia to the decapitated stem. The 

 orchid pollinia are, of course, rich in auxin. The treated 

 stems frequently showed swelling or elongation or both, 

 and this led Laibach and Miiller to postulate the mechanism 

 of inhibition discussed below. 



If a leaf of Bryophyllum be cut off and replaced on its 

 petiole, the bud in the axil continues to be inhibited, and 

 the same result may be obtained by placing the leaf on agar, 

 and then applying the agar block to the cut petiole (Uhrova, 

 1934). There is no reason to doubt that here, too, auxin is 

 the active factor. In Solidago, the successive leaves of the 

 rosette each inhibit the growth of the one following them for a 

 time. Goodwin (1937) showed that the inhibiting power of 

 the leaf ceases about the time that its auxin production 

 falls off. Further, removal of one leaf accelerates the growth 

 of the next, while application of pure auxin on the cut 

 petiole inhibits the growth of the next. Apphcation of 

 pure auxin inhibits bud development in the Aster. In this 

 plant the much-branched habit of A. multiflorus is correlated 

 with very small auxin production by the buds, while the 

 almost unbranched A. novae-Angliae produces considerably 

 more (Delisle, 1937). We may safely assume, therefore, that 

 the inhibiting action of auxin on bud development is very 

 general. 



Not only is the phenomenon common to a great many 

 different plants, but it is also caused by a great many chemi- 

 cally different auxins. The natural auxin of the plant, pure 

 auxin h, and indole-acetic acid all have the same effect. 

 Further, indene-3-acetic and coumaryl-1-acetic acids, both 

 of which have other properties of auxins, also strongly 

 inhibit bud development of Pisum seedlings (Thimann, 

 19356). Hitchcock (1935a) mentions that indole-propionic 

 and naphthalene-acetic acids had bud-inhibiting influence 

 when applied in high concentrations to Nicotiana stems. 



